Sinn Fein, the SDLP and cross community Alliance Party are all campaigning for the UK to remain within the EU.

The Democratic Unionists are backing a withdrawal from Europe.

The Ulster Unionists have delayed nailing their colours to the mast on the issue until they have an internal debate on Mr Cameron's EU reform deal, with leaders expected to make the party's position clear next week.

However, UUP leader Mike Nesbitt has already made a number of pro-EU remarks in recent days.

Earlier this week, Northern Ireland Secretary Theresa Villiers also faced calls from nationalists over her stance on the Brexit issue.

However, the Prime Minister has pledged his support, telling the Commons Ms Villiers does an "excellent job".